Porous electrodes and so-called showerhead counter electrodes have been used for plasma etching semiconductor wafers. Such systems have included a reactor chamber including a pair of electrodes across which a source of power is applied. A silicon wafer, which may have a metal film thereon to be etched is generally placed on one of the electrodes. The gas for producing the plasma gas within the reactor is generally introduced into the reactor through the porous or showerhead electrode.
While porous electrodes are satisfactory in many cases, they present some problems in others. One of the problems with porous plate electrodes is there is a limited range of materials which can be fabricated in porous forms and not every material is available in porous forms. The range of pore sizes is sometimes limited for given materials. Therefore the gas conductance through the porous electrode may be limited. In some processes, the pores in the electrode are easily clogged up.
The surface area of exposed surface material is very large in a porous plate. Therefore, any chemical reaction which may occur between the electrode material and the etching gases could lead to corrosion or other effects.
A showerhead electrode comprises an electrode having many holes drilled in a solid plate. Those kinds of electrodes generally work very well. However, in order to make them work well, it is necessary that the holes be very small, specifically under about 0.020" diameter.
If the holes are too big, discharges will form in the holes, and the edges of the holes will become eroded away and the electrode is subject to hot spots and becomes damaged. On the other hand, if the holes are made very small, two things may happen: First, it is difficult to fabricate the holes and costly techniques such as laser drilling may be required especially if the electrode is a hard material. Second, if the holes are very small, the number of holes required becomes very large to obtain sufficiently low gas velocity through the holes. The gas flow velocity depends on the total cross-sectional area of the holes. If the velocity becomes too high, print-through effects result. An etch non-uniformity pattern corresponding to the showerhead hole pattern may appear. At trans-sonic gas velocities, severe print-through effects occur.
To successfully use the showerhead, it is often necessary to increase the gap between the electrode and the wafer to dissipate any print through effect. A large gap, however, may mean that additional power is required.
Another problem that is common both to showerheads and to porous plate electrodes is that often the pressure drop across the electrodes can be relatively small, particularly for small gas flow rates. That is, the pressure behind the electrode used may be scarcely higher than the pressure in the process. Now that would not normally be a problem except that in general it is difficult to get sufficient heat transfer from the electrodes to the liquid cooled thermoplate. One of the easiest ways to achieve good heat transfer and therefore control the temperature of the electrode is to have a gas film between the electrode and the thermoplate. And the higher the pressure of that gas up to a point, the better the heat transfer.